1. Field of the Invention
The present invention relates to a lens holder to be fixedly bonded to a certain other fixing member after a lens array or an elongated lens is fixedly bonded thereto. The present invention also relates to a laser array unit using such lens holder.
2. Description of the Related Art
A laser unit in which a plurality of laser beams are inputted to a single fiber to combine the beams in order to obtain a high power laser beam is known as described, for example, in U.S. Pat. No. 6,995,912 and Japanese Unexamined Patent Publication No. 2004-134641. The laser unit is basically constituted by a semiconductor laser array (which may be an integrated unit or constituted by a plurality of discrete elements) having a plurality of luminous points to emit a plurality of laser beams; a single optical fiber; and a condensing optical system for condensing and coupling the plurality of laser beams emitted from the semiconductor lasers to the optical fiber.
In such type of laser unit, optical components, including a collimating lens, a condenser lens, and the like constituting the condensing optical system, a lens holder for holding these components, and the like are generally very small. For fixing these small components to the main body of the laser unit, a fixing structure using an adhesive is widely used as described, for example, in U.S. Pat. No. 6,995,912.
In the mean time, it is customary that the semiconductor laser array is fixed on a heat block formed of a material such as copper having a high thermal conductivity directly or through a submount, so that the heat generated by the semiconductor laser array is dissipated efficiently. When such heat block is used, it is often the case that the collimating lens array for collimating the laser beams emitted from the semiconductor laser array is fixed to a lens holder of such kind as described above, and the lens holder is fixed on the heat block. In this configuration, a UV cure adhesive is often used for fixing the collimating lens array to the lens holder, in which case the lens holder is generally formed of transparent glass so that a sufficient amount of UV rays is irradiated on the area to be bonded.
As described above, when the lens holder formed of glass is fixed to the heat block formed of a metal, an adhesive is generally used. In such case, it has been a problem that it is difficult to secure both high bonding strength between the lens holder and heat block, and high positional accuracy for fixing the collimating lens array to the lens holder. Hereinafter, such problem will be detailed by taking, for example, the beam combining laser module disclosed in U.S. Pat. No. 6,995,912 as an example. That is, the problem often found in a beam combining laser module in which each of a plurality of laser beams emitted from the semiconductor laser array is collimated by each of the collimating lenses of the collimating lens array, condensed by a single condenser lens, and coupled to the optical fiber will be detailed hereinbelow.
In the beam combining laser module described above, if the magnification of the condensing optical system constituted by the collimating lens array and condenser lens is 7.8×, the movement of the laser beams at the lens section is magnified by 7.8 times on the input face of the optical fiber. Here, it is preferable that the movement of the leaser beams on the input face of the optical fiber is not greater than 1.5 μm in order for the laser beams to be effectively coupled to the optical fiber with a core diameter of, for example, 60 μm when positional variations of the lens and semiconductor laser array are taken into account. In addition, when the magnification of the optical system described above is taken into account, the relative movement between the lens system and semiconductor laser array should be within approximately 0.2 μm.
Further, for the collimating lens array in which a plurality of collimating lenses is arranged side by side, if the array moves around the center of the array in the longitudinal direction (lens arrangement direction), the outermost collimating lenses move the most. In order to keep the relative positional movement between the outermost collimating lenses and semiconductor laser array less than approximately 0.2 μm, the rotational movement described above should be kept within 0.2′ (arcmin) when the length from the center of the array to the outermost collimating lenses is 3.6 mm.
It is known that an organic adhesive, such as a UV cure adhesive or the like shrinks when solidifying. Consequently, if the difference in thickness or angle of the adhesive occurs between the collimating lens array and lens holder, the inclination in the rotational direction described above may occur through the shrinking solidification. In order to keep such movement of the collimating lens array within 0.2′, the collimating lens array and lens holder should be bonded together with a face alignment accuracy (accuracy in setting two bonding surfaces in parallel with each other) of within approximately 2′ between them when considering the volume shrinkage of the adhesive which is generally around 10%.
The face alignment accuracy between the collimating lens array and lens holder depends on their positional measurement accuracy and surface irregularity. If the surface irregularity of the bonding surface is fixed, then a longer bonding surface may provide a higher face alignment accuracy.
In the mean time, the collimating lens array is generally formed of transparent glass, such as BK7 glass or plastic, and it is customary that the lens holder for fixing the array is also formed of the same or similar material in order to improve the surface irregularity of the surface to which the collimating lens array is bonded and fixing accuracy.
On the other hand, the heat block to which the semiconductor laser array is fixed is formed of a metal, such as copper or the like in order to improve heat dissipation efficiency. When the lens holder formed of glass or the like is fixedly bonded thereto, therefore, the thermal expansion coefficient differs greatly with each other. Consequently, strain is developed in the bonded section between the heat block and lens holder when subjected to changes in ambient temperature or temperature changes arising from activation and deactivation of the semiconductor laser array. The amount of strain is dependent on the maximum length of the bonded section, and when the strain grows significantly, the bonded section may unstick.
Conventional lens holders are shaped in a rectangular solid, so that if they are made larger in order to improve the face alignment accuracy with the collimating lens, then the bonding surface of the holder with the heat block inevitably becomes lengthy, which leads to unsticking of the bonded section. On the other hand, if they are made smaller in order to avoid the unsticking of the bonded section, the face alignment accuracy is degraded.
The above description is a problem that may occur in the case where a lens array is fixedly bonded to a lens holder, and then the holder is fixedly bonded to a heat block. But a similar problem may occur in the case where an elongated lens, such as a cylindrical lens or the like is fixedly bonded to a lens holder instead of the lens array, or where the lens holder is fixedly bonded to a fixing member other than the heat block. Further, even if the fixing member such as the heat block and the lens holder have the identical thermal expansion coefficient, if the adhesive layer between them has a high heat insulating property, which is normally the case, the same problem of strain development may also occur when subjected to the temperature change.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a lens holder capable of securing a high face alignment accuracy with a lens array or an elongated lens, and minimizing the strain in the bonded section with a certain other fixing member arising from the temperature changes thereof.
It is a further object of the present invention to secure a high face alignment accuracy between a lens array or an elongated lens and a lens holder for fixedly bonding the lens array or elongated lens thereto, and to minimize the strain in the bonded section between the lens holder and a certain other fixing member in a laser array unit constructed such that a plurality of laser beams emitted from a laser array is received by the lens array or elongated lens.